How camera size affects render quality?
The Camera Size in the 3D viewport will make no difference in the qualities of the rendered image. Changing the size is meant to help you find and visualize the camera while you set the scene.
The Sensor Size will make a difference in the characteristics of the image, if it used in combination with the Lens Size and f-Stop to control depth of field (meaning how sharp, or blurry, the objects will be if they are closer to the focus distance or further away from it).
The Render Size in pixels, or resolution will determine how large the image will be, and how it is represented in a monitor (or whatever other media is used to display it). The more pixels the image has will have an impact on the amount of detail and how large the image can be displayed, or printed, before it is no longer sharp.